The legible body and the language of cognition.

Neural activity moves the body to act on the world, and the world returns through the body to shape neural activity. Systems neuroscience built tasks to cut this loop, isolating low-dimensional, task-related signals and discarding the rest. Tasks suited cognition as classically conceived: general-purpose machinery between sensation and action, performing latent computations abstracted from specific behaviors. With recording techniques limited to tracking individual neurons and discrete actions, it took tasks to make cognition legible by supplying meaning within a limited but generalizable frame. That constraint has been lifting: brain-wide neural recording and rich video tracking of the body can now read neurons and behavior as two facets of one dynamical system. Together they reveal cognitive primitives - decision variables, expected values, hidden states - distributed not only in brain activity but also in subtle face and body movements long dismissed as noise. Cognition, it turns out, is not confined to higher-order cortex; it surfaces in incidental movements carrying latent variables the brain computes - even ones it never acts on. It keeps its hold on the body, as emotion keeps its expressions. Backed by normative theories, noninvasive video recordings can divulge not only the syntax of behavior but also its semantics, opening the study of cognition in the wild beyond tasks. The potential is perhaps greatest in our own species, where the promise of reading the mind noninvasively - to treat and to understand - is mirrored by the threat of laying it bare.